It is often necessary or desirable to determine the location of a medical probe relative to a location of interest within three-dimensional space. In many procedures, such as interventional cardiac electrophysiology therapy, it is important for the physician to know the location of a probe, such as a catheter, (especially, a therapeutic catheter) relative to the patient's internal anatomy. During these procedures, a physician, e.g., steers an electrophysiology mapping catheter through a main vein or artery into the interior region of the heart that is to be treated. The physician then determines the source of the cardiac rhythm disturbance (i.e., the targeted cardiac tissue) by placing mapping elements carried by the catheter into contact with the heart tissue, and operating the mapping catheter to generate an electrophysiology map of the interior region of the heart. Having identified the targeted cardiac tissue, the physician then steers an ablation catheter (which may or may not be the same catheter as the mapping catheter above) into the heart and places an ablating element carried by the catheter tip near the targeted cardiac tissue, and directs energy from the ablating element to ablate the tissue and form a lesion, thereby treating the cardiac disturbance.
Traditionally, navigation of catheters relative to points of interest has been accomplished using fluoroscopy. In this case, radiopaque elements are located on the distal end of the catheter and fluoroscopically imaged as the catheter is routed through the body. As a result, a two-dimensional image of the catheter, as represented by the illuminated radiopaque elements, is generated, thereby allowing the physician to roughly determine the location of the catheter. The use of fluoroscopy in locating catheters is somewhat limited, however, in that the physician is only able to visualize the catheter and surrounding tissues in two dimensions. In addition, fluoroscopy does not image soft tissues, making it difficult for the physician to visualize features of the anatomy as a reference for the navigation. Thus, fluoroscopy is sub-optimal for the purpose of navigating a catheter relative to anatomical structure composed primarily of soft tissues, e.g., within the heart.
Various types of three-dimensional medical systems (e.g., the Realtime Position Management™ (RPM) tracking system, developed commercially by Boston Scientific Corporation and described in U.S. Pat. No. 6,216,027 and U.S. patent application Ser. No. 09/128,304, entitled “A Dynamically Alterable Three-Dimensional Graphical Model of a Body Region,” and the CARTO EP Medical system, developed commercially by Biosense Webster and described in U.S. Pat. No. 5,391,199), have been developed, or at least conceived, to address this issue. In these medical systems, a graphical representation of the catheter or a portion thereof is displayed in a three-dimensional computer-generated representation of a body tissue, e.g., a heart chamber. The three-dimensional representation of the body tissue is produced by mapping the geometry of the inner surface of the body tissue in a three-dimensional coordinate system, e.g., by moving a mapping device to multiple points on the body tissue. The position of the device to be guided within the body tissue is determined by placing one or more location elements on the device and tracking the position of these elements within the three-dimensional coordinate system. An electrophysiological map generated from information acquired by the mapping device can also be graphically displayed.
The main difference between the RPM tracking system and the CARTO EP system is that the latter establishes an external coordinate system using magnetic transmitters located outside of the patient's body, whereas the former establishes an internal coordinate system using ultrasound transceivers mounted on reference catheters that are located within the heart itself. Because the measurements taken by the CARTO EP system are performed in an external coordinate system, magnetic sensors must be attached to the patient or within the patient, such as on a reference catheter placed within the heart, so that inadvertent movement of the patient (e.g., movement caused by respiration and/or patient shifting (heart pumping is compensated by gating)) may be compensated for when measuring the location of the mapping device and other devices to be tracked. In contrast, because the measurements taken by the RPM tracking system are performed in an internal coordinate system, which self-compensates as the patient moves (i.e., the internal coordinate system moves as the patient moves), no additional sensors are needed.
Although the uses of the RPM tracking system and CARTO EP system have, to a large extent, been successful in facilitating the navigation of catheters within the cavities of the heart, the graphical representations of the heart chambers lack the resolution produced by conventional imaging systems, such as Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) systems. This lack of resolution especially impacts the navigation of catheters within and around the complex anatomy of the left atrium and pulmonary veins. In order to more accurately localize catheters, enhance the efficacy of ablative lesions, and reduce procedure time, it has been suggested that high resolution preoperative images, such as those generated by CT and MRI systems, could be merged with three-dimensional graphical information, such as electrophysiological maps and catheter representations.
For example, Biosense Webster has developed a software package, referred to as Cartomerge™, which provides the CARTO EP system with the capability of merging three-dimensional CT images with electrophysiological mapping data. This integrated system requires the user to merge the CT image with the electrophysiological map by matching corresponding anatomical reference points on the image and map. Besides requiring an additional step to be performed by the user, the image integration is only as accurate as that of the anatomical envelope of the electrophysiological map, which as discussed above, lacks the resolution typically seen in a CT image. As such, catheter localization relative to the CT image may be inaccurate to a certain extent. Also, this system requires some type of graphical representation of the heart to be displayed prior to the image merging step, so that the anatomical reference points can be matched. Thus, the CT image cannot be used when initially navigating the catheter within the heart. In addition, integration of the CT image is accomplished within an external coordinate system, thereby requiring additional sensors and/or catheters to compensate for patient movement.
There thus remains a need for a self-compensating system and method that automatically integrates a medical image, such as a CT or MRI image, with a graphical information, such as an electrophysiological map and/or catheter representations.